The present disclosure is related generally to turbochargers having a variable nozzle for regulating flow into the turbine, wherein the position of a variable-geometry member of the nozzle is adjusted by an actuator in order to adjust the flow rate through the nozzle. The disclosure is related more particularly to a method for calibrating the actuator to improve the accuracy and repeatability with which the flow rate is set.
Turbochargers are used for boosting the performance of internal combustion engines, by compressing the air before it is supplied to the engine air intake, thereby increasing the air mass flow rate through the engine. In an exhaust gas-driven turbocharger, exhaust gas from the engine is passed through a turbine, which drives a centrifugal compressor for compressing the intake air. In many turbocharger systems, the turbine includes a variable nozzle having a variable-geometry member that is movable for regulating the effective flow area through the nozzle and hence the flow rate of the exhaust gas being delivered to the turbine. Various types of variable nozzles have been developed, including sliding pistons as well as vanes that are variable in setting angle. In most such variable nozzles, an external actuator is coupled with the variable-geometry member for causing movement thereof. The actuator is controlled in some fashion in order to adjust the variable-geometry member's position as needed for establishing the desired flow rate for a given engine operating condition.
Governmental regulations limiting the amount of emissions from internal combustion engines are growing increasingly stringent. One of the factors affecting emissions is the flow rate through the turbine of the turbocharger. Accordingly, it is desirable to be able to control the turbine flow rate with accuracy and repeatability. This, in turn, requires controlling the position of the variable-geometry member of the nozzle accurately and repeatably.
One of the difficulties encountered in controlling the nozzle flow rate is the inevitable variability in nozzle flow characteristics from one turbocharger to another. Even when two turbochargers are manufactured based on the same manufacturing specifications, they can have different nozzle flow characteristics because of manufacturing tolerances. As a result, if the actuator systems of the turbochargers are commanded to move to the same actuator positions, the resulting flow rates through the nozzles can differ, and sometimes the difference can be appreciable.
For these reasons, it is known to perform a calibration for each turbocharger/actuator pair in order to determine how to set the actuator to achieve a desired nozzle position. Such calibration can help to reduce at least some of the variability and thereby improve accuracy and repeatability of nozzle flow rate from one turbocharger to another.